In most cases, the actuator used is a screw actuator and the body can be in two telescopic portions. The larger-diameter portion carries a screw that is prevented from moving in translation relative to the said portion while the smaller-diameter portion carries a nut, said portion being prevented from rotating relative to the larger-diameter portion. By driving the screw via the end of the actuator body, it is thus possible to extend or to retract the body, and thus to move the cups which are fixed thereto either away from or towards each other.
There also exist fluidized pressure actuators that can be single-acting or double-acting, for providing the above relative displacement. Particular mention can be made of the actuator described in document U.S. Pat. No. 4,034,960. In that document, the spring-compressor has two actuators side by side, each having a fixed piston and a moving cylinder, one cup being secured to the moving cylinder of each the actuators, while another cup is carried by a plate which is fixed relative to the piston and which is secured to a guide tube for guiding each of the moving cylinders relative to the piston. The presence of the two actuators side by side is necessary firstly to enable sufficient compression force to be developed, given that the device is a pneumatic device, and secondly to provide rectilinear guidance for the moving cup relative to the fixed cup so as to oppose the tendency of the spring to tilt one cup relative to the other. Finally, the forces involved are such that to oppose buckling, the cylinder of each pneumatic actuator must remain well within the corresponding guide tube when the device is at its maximum extension. This means that the axial size of the apparatus is large for given stroke. All of those reasons that give rise to large bulk mean that that apparatus is ill-adapted, particularly to taking action beneath a motor vehicle body when changing or repairing a shock absorber.